1.00/1.001 Introduction to Computers and Engineering Problem Solving

Recitation 2 Iteration and Methods

Spring 2012

1

Quick Exercise: Increment Operator

int a = 1;

int b = a + a++;

What is the value of a and b at the end of each code fragment?

int a = 1;

int b = a + ++a;

int a = 1;

int b = a++ + a;

int a = 1;

int b = a*++a;

int a = 1;

int b = (a++ + 2)*a;

int a = 1;

int b = a++ + a + a++;

2

Iteration (Loops)

while (condition to continue)

{

// repeat as long as condition = true

}

do

{

// run once and repeat as long as condition = true

}

while (condition to continue);

for (initial statement, condition to continue, increment statement)

{

// execute initial statement

// repeat as long as condition = true

// execute increment statement after each repetition

} 3

while vs. do...while

double weight = 0;

while (weight < 40) {

String s = JOptionPane.showInputDialog("Enter Weight");

weight = Double.parseDouble(s);

}

double weight;

do {

String s = JOptionPane.showInputDialog("Enter Weight");

weight = Double.parseDouble(s);

}

while(weight < 40);

// Declaration and Initialization

// Declaration only

NO semicolon!

semicolon! 4

while vs. for while loops and for loops are often interchangeable.

What does the following loop do?

int i = 0;

while (i < 5) {

System.out.print(i + " ");

i++;

}

How would you do the same thing with a for loop?

5

Common for loop errors (Check these first if you get

unexpected results)

for (int i = 0; i > 5; i++); {

System.out.println(i + " ");

}

Common errors in declaring for loops: 1) Incorrect termination statement

• Remember, the loop will only run as long as the termination is TRUE • Make your termination statement to be true as long as you want the loop to run,

not when you want it to stop

2) Semicolon after loop declaration • The for loop and all loops exist inside brackets – brackets are a way of grouping

bits of code for execution • The semicolon terminates a line. Java will move to the next line and won’t know

to associate the for loop with what is in the brackets

Empty statement!

What does this code do?

Terminates immediately!

6

while vs. for The method uses a for loop to raise x to the power of n. public static double power(double x, int n) {

}

Write a while loop version public static double power(double x, int n) {

}

double result = 1;

for (int i = 1; i <= n; i++)

result *= x;

return result;

What does this code do?

7

Nested Loops What does the following double loop print?

int i = 0;

while (i < 3)

{

i++;

for (int j = 0; j <= i; j++)

{

System.out.println(10*i + j);

}

}

Can you get rid of the some braces {} and keep the same output?

8

Methods

What is a method?

Where do you write methods?

How many methods per class?

How many main() method(s) per class?

9

Method Signature

Example: a power method that raises any number x to an integer power n

public static double power(double x, int n)

for now, take public static

as a template

Return type (output type)

Name Arguments

(input)

• A method has a single return type: it can only return one "thing." • If nothing is returned, the return type is void. • The number of arguments is not limited. 10

Exercise: Income Tax Calculator

Income Bracket Tax rate (%)

0 – 5,000 0 5,000 –12,500 7

> 12,500 16

For example, if income = 13,000: tax = 0.07 * (12,500 - 5,000) + 0.16 * (13,000 - 12,500)

Using the following tax brackets, write a method to compute the tax applicable to any income.

Method signature?

11

Exercise: Income Tax Calculator

public static double calcTax(double inc) {

if (inc < 5000)

return 0;

else if (inc < 12500)

return 0.07 * (inc - 5000);

else

return 0.07 * (12500 - 5000) + 0.16 * (inc - 12500);

}

What if you use an else if instead of an else for the third case?

12

Passing Arguments to Methods

public static void main(String[] args) {

double a = 2.0;

int b = 3;

double p = power(a, b);

}

public static double power(double x, int n) {

double result = 1.0;

while (n > 0) {

result *= x;

n--;

}

return result;

}

2.0 3 8.0

Primitives are passed "by value".

13

Exercise: Methods

You will write a method to compute binomial coefficients. The binomial coefficient can be computed as: Where x! is the factorial operator (e.g. 5! = 5 * 4 * 3 * 2 * 1)

!)!(

!

kkn

n

k

n

k

n

Strategy?

14

Factorial Method

// for loop version

// while loop version

15

Binomial Method

// putting it all together

Using methods avoids code repetition!

!)!(

!

kkn

n

k

n

16

Homework 2: Throw ball in basket

Main method should:

• Prompt user for input

• Call the following methods: – Compute optimal angle

– Compute smallest angle

– Compute largest angle

– Compute max height

– Determine if ball hits ceiling

– Adjust velocity incrementally until ball does not hit ceiling anymore

• Print all results to console

Math is an existing library in Java Example methods in Math class: # = Math.cos() = Math.asin(#) xy = Math.pow(x,y) = Math.PI

Math methods are in radians!

h

d

vo

θ

x

y

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17

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